Silicon Valley Computer SHUGART 706 User Manual download pdf (Page 31)

The

circuit

A schematic

of the

circuit is

shown

in Fig. 1. The input

section

determines whether

the logic

level is high

or low,

and enables

the appropriate

tone

generator; it

consists

of two sections

of an

LM339

quad

comparator.

One of

the

comparators (IC1 -a)

goes high

when

the input voltage

exceeds

67%

of the supply voltage.

The

other comparator

goes high when

the input

drops below 33%

of the

supply.

Resistors R1

and R2 ensure

that neither

comparator

goes high

when

the input is floating

or be-

tween the

threshold levels.

The tone generators

consist

two

gated astable

multivibrators.

The generator built

around

IC2 -a

and

IC2

-b produces the high tone.

The one built

around

IC2 -c and

IC2 -d

produces the low tone.

Two

diodes, D1 and D2,

isolate

the tone

-generator outputs.

Tran-

sistor Q1 is used to

drive

a low -

impedance

speaker.

The tester can be

built

small

piece

perforated

construction

board; a

board could also be

used, if

desired.

Layout is not

crit-

ical. The

completed board should

be mounted in a

case.

The size

its

case will most likely be

deter-

mined

by speaker's size.

Two notes

on the IC's: The 4011 is

a CMOS device. As with all other

CMOS devices, extra care should

be exercised in handling to

avoid

subjecting the

to damaging stat-

electricity.

Also,

all unused

in-

puts on the LM339 should be tied

to ground.

The

tester

designed to draw

its

power from

the circuit under test.

Therefore, the power and

ground

leads

should be terminated by

small alligator

clips. Connect

spare

multimeter probe

the

tester's input

and you are all

set.

Connect the power

-supply lead

to the positive supply

rail

of the

circuit under test. Connect the

ground lead to the circuit- under-

test's ground rail. At this

point,

the

tester should be silent.

Now,

touch the probe tip to the circuit's

positive supply

rail. The

tester

should

produce a

high -pitched

tone. Next,

touch the probe to

the

ground rail. The

tester should pro-

duce a low- pitched tone.

all

well, your

tester

is ready for

use.

-Philip L.

Kane

Analyze

defective

waveforms

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Make

only

one

circuit connection and push

one button

for

each

circuit parameter test:

You

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peak -to -peak

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rise

fall

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voltage equivalents be-

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microseconds.

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logic circuit testing.

Analyzing

troublesome divide

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no time -consuming

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Silicon Valley Computer SHUGART 706 User Manual Page 31

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